**Al-Cr-B-N**

(AlCr)N and (AlCrB)N coatings were arc deposited using Al70Cr30, Al60Cr40 and B-doped cathodes with a constant Al/Cr atomic ratio of 1.8 and varying B content of 10, 20 and 30 at.% [89]. The resulting coatings had a B content between 2.3, Al26.7Cr21.7B2.3N49.3, and 9.1 at.%, Al22.7Cr20.5B9.1N47.7, corresponding to (Al + Cr)/B ratios in the range 0.04– 0.21. The authors stated that even at low B contents, B segregates to the grain boundaries where (AlCrB)N crystallites are at least partially covered by an a-BN*<sup>x</sup>* tissue phase. A nanocomposite structure was formed at compositions of ≥5.7 at.% B, consisting of fcc- (Al,Cr,B)N grains surrounded by an amorphous BNx phase dependent on the B content. The hardness was significantly increased from ca. 30 GPa for coatings containing no B to a maximum of about 40 GPa for a B content of 5.7 at.%. The addition of B acts as a grain refiner where the highest B content of 9.1 at.% reduced the grain size down to 5 nm. It was also shown that the addition of B decreases the coating stress.

A similar investigation of arc deposited coatings was carried out using cathodes of Al70Cr30 and Al55Cr35B10 [58]. The (AlCrB)N coating contained 2.1 at.% of B for Al30.4Cr16.3B2.1N48.1O3.1, with (Al+Cr)/B = 0.045 and Al/Cr = 1.86, and was reported to be composed of an fcc solid solution when measured by XRD. XPS measurements showed peaks of B-N and Cr-B bondings. Thus, a BNx tissue phase formation is highly probable. The B alloying significantly increased the hardness from the B-free Al33.1Cr15.8N48.1O3 at 31 to 37 GPa. The grain size concurrently decreases from 16 to 5 nm. The friction value and the wear rate measured by pin-on-disc were lower than that of the coating with no B.

(AlCrB)N coatings have been deposited by the cathodic vacuum arc method using cathodes of Al70Cr25B5 and Al70Cr30. The (AlCrB)N coating had an Al/Cr ratio of 1.96 and a B/(Al + Cr)/ratio of 0.04. Oxidation tests at 900 ◦C showed that this particular (Al64.5Cr32.9B2.6)N coating had a lower oxidation stability than the AlCrN coating [81].

In general, it can be concluded that the addition of B decreases the grain size in combination with an increase in hardness of around 30%.

4.4.2. Coatings with the Addition of Si

Si addition is the most investigated alloying element, with the goal of achieving an improvement of mechanical properties and oxidation resistance [72,117–121]. The properties are mainly dependent on the Si content at a given Al/Cr ratio, and also on the deposition technology. The Al/Cr ratio and the addition of Si have to be optimised to keep the results primarily in the structural zone of fcc-CrAlSiN or fcc-AlCrSiN so that suitable mechanical properties can be achieved, particularly for applications on cutting tools.

Figure 20 shows a schematic representation of the structural evolution as a function of the Al/Cr ratio and Si content. In a first approximation, the limit of the Al content to obtain a coating dominated by the fcc phase is assumed to be about the same as for Si-free coatings (ca. 70 at.% Al).

**Figure 20.** Schematic diagram of the phase evolution for (CrAlSi)N and (AlCrSi)N for different Al and Si contents in the coating, nc = nanocomposite.

However, it was shown that the addition of Si promotes the formation of the hcp phase. Arc-deposited coatings use a constant Al content of 70 at.% in the cathode, but with a different Cr and Si content (0, 1, 2, 5 at.% Si) [72]. For coatings synthesised at low bias voltages (40 V), a phase separation into fcc (Al,Cr,Si)N and hcp (Al,Cr,Si)N for Si contents of around 1 at.% in the coating, e.g., Al33Cr16Si1N51, was observed. This results in a congruent drop in mechanical properties. The hardness decreased from 30 ± 2 GPa for Al32Cr17N51 coatings to 21 GPa for the coating Al32Cr14.5Si2.5N51 deposited using Al70Cr25Si targets. Higher bias voltage favoured the fcc phase formation. However, it is unclear if this is due to growth effects or due to a decrease in the Al and Si contents as a secondary effect of the bias voltage [72].

AlCrSi cathodes with a constant Cr content of 30 at.%, but with varying Al and Si silicon concentrations (0, 1, 2, 5, 10 at.% Si), were deposited by cathodic arc evaporation [73]. The Al/(Al + Cr) ratio of the coatings decreased from about 0.68 to about 0.64 at the highest Si content. Up to a Si-content of about 1 at.% (corresponding to cathodes with 2 at.% Si), the coatings showed an fcc structure, Al33.9Cr18.1Si1.1N51O0.9. The coating with a composition of about Al33.4Cr18.3Si2.3N46O0.7 (5 at.% in the cathodes) displayed the formation of the hcp phase at an Al/(Al + Cr) ratio of only about 0.65 and a Si content of about 2 at.%.

Investigations of whether the addition of Si decreases the phase stability of the fcc phase at higher temperatures are lacking. In addition, it is also not clear how much Si can be substitutionally incorporated into (CrAl)N and (AlCr)N at a given Al content, and what the critical Si content is for the segregation of an amorphous SiNx phase at the grain boundaries.

Above the solid solubility limit of Si, a nanocomposite (nc) structure is formed [35,121–127]. The Si segregates along the grain boundaries, thus an amorphous SiNx phase grows. This effect has been observed for both (CrSi)N and (AlSi)N. A schematic structural model for (CrSi)N was shown in [128]. It was determined that for (AlSi)N, the tissue phase is generated for (AlSi)N in a coating containing 4 at.% of Si, Al46Si4N50. The authors showed that the nanocomposite was formed at 6 at.% Si in the coating for Al44Si6N50 [129].

A nanocomposite structure was also detectable for (CrSi)N with a Si content of about 2 at.% or more [130]. Analogous phase evolution was shown for coatings with a low aluminium content (Cr/Al ca. 3) for (CrAlSi)N [35]. An (AlCrSi)N coating with a nanocomposite structure was demonstrated at a Si content of 3.5 at.% [124].

Unfortunately, no systematic investigations of the minimum Si content for different Al contents in a (CrAlSi)N or (AlCrSi)N coating are available. Roughly, it can be deduced that a nanocomposite structure might be generated at a Si content of between 2 and 4 at.% of the total chemical composition (Al + Cr + Si + N content in at.% equal to 100 at.%). It was shown that the addition of Si not only has an influence on the thermal properties of (AlCr)N, but also acts as a grain-size refiner. Depending on the amount of Si, the grain size is in the range of less than 10 nm [124,131].

A comparison of the oxidation behaviour of (AlCr)N (Al/Cr = 1.25), (CrAlSi)N (Cr/Al = 1.16) and (AlCrSi)N (Al/Cr = 1.89) coatings revealed that the coating (AlCrSi)N with 3.3 at.% Si, Al32.7Cr17.4Si3.3N44.3O2.3, showed the highest oxidation resistance [80]. Additional work was performed to investigate the oxidation of various Si-free and Sicontaining coatings [119]. Figure 21 shows the specific weight gain of fcc-structured (CrAl)N, (CrAlSi)N and (AlCr)N, (AlCrSi)N coatings measured using TGA in synthetic air. The authors showed that the formation of a crystalline corundum type mixed and/or layered (Al*x*Cr1−*<sup>x</sup>*)2O3 oxide scale, composed of Cr-rich and Al-rich areas, is typical for (CrAl)N, (AlCr)N and (CrAlSi)N and (AlCrSi)N coatings for coatings with low Si contents. The oxide layer formed prevents further oxidation. A special case was observed for high Si contents, e.g., a (Cr45Al39Si16)N coating, where the highest onset temperature for oxidation was observed accompanied with the lowest weight gain. The reason is that a crystalline SiO2 phase grows in addition to (Al*x*Cr1−*<sup>x</sup>*)2O3. This effect might be used for some coating solutions requiring a low oxidation rate.

**Figure 21.** Weight gain measured by dynamic TGA measurements of fcc-structured (AlCr)N, (Al-CrSi)N and (CrAl)N, (CrAlSi)N coatings up to 1440 ◦C in synthetic air, redrawn after [119], original © American Vacuum Society.

The (CrAlSi)N and (AlCrSi)N coatings can also be doped with one additional element to address modifications of various properties, e.g., O [132], Y [133]B[134], W [135,136], Ni [137], and others. Additionally, more than one element was added, e.g., Y and O [138]. Multilayer architectures were built using CrAlSiN and AlCrSiN combined, for example, with Si-free AlCrN layers [139], with AlSiN [140], or with coatings containing Si, such as Cr-doped AlSiN [127], with MoN, NbN [139], or with CrN [141], and others.
